I think this has something to do with Microsoft's mission to deprecate Windows Registry settings that 3rd party apps use to alter or restore the Windows 10 Taskbar. I'm really disappointed with this change. Hopefully someone figures out a way to restore the ability to enlarge the taskbar thumbnail preview size.
EDIT: After further testing, I have found that none of the following Registry additions work anymore.
Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Taskband
NumThumbnails; MinThumbSizePx; nor MaxThumbSizePx